"Snap-ring" plastic tamper-evident closures are not new per se. The prior art as typified by Mumford et al U.S. Pat. No. 4,432,461 and Fields U.S. Pat. No. 3,329,295 show container closures of the general type to which the present invention relates. These closures typically comprise a cap of cup-like form having internal threads which cooperate with threads on a container finish and a tamper-evident band or ring depending from the lower terminal edge of the shirt portion of the cap and connected thereto by a series of circumferentially spaced fracturable bridge connections. The tamper-evident band usually includes locking means in the form of a rib which engages under a radially outwardly directed bead or flange below the threads on the container. By this construction, the cap may be applied to the container in the usual manner by threading to a position where the tamper-evident band expands to allow its inwardly directed rib to "snap" over the bead or flange on the bottle finish. These closures are referred to herein as "snap-ring" type closures. Now when the cap is rotated to remove it from the container, the rib and bead interengage to effect fracture of the bridges permitting removal of the cap portion and leaving the break-away band attached to the container as evidence that the cap has been once removed and the container seal broken. Typically, these closures, including the bridges, are molded of plastic material in a conventional plastic molding operation.
The patent to Hannon No. 3,861,551 is of interest in that it shows a method for slitting and scoring metal caps in preselected areas.
One of the drawbacks and disadvantages of the prior art tamper-evident closures described above is related to the mechanics of application. Ideally, on application the tamper-evident band expands uniformly over the bottle restriction or bead. However, some factors prevent this ideal from actually being achieved. The geometric configuration of the bottle finish bead can be a major reason for non-uniform ring expansion. Container neck ovality, profile variations, variations in clearnaces and thread engagement also contribute to non-uniform band expansion. In addition, because the band and cap skirt are connected by flexible bridges, the band can shift in a direction perpendicular to the axis of the cap referred to herein as "lateral shifting" as illustrated in FIGS. 16-18 inclusive. In poorly designed systems, the above deviations from the ideal can cause application problems as follows: (1) "telescoping" of the band up around the cap skirt; and (2) excessive lateral shifting of the ring and the resultant adverse affects. These affects can best be described and understood with reference to the various illustrations in the drawings and particularly FIGS. 16-18 which illustrate common closure mechanics of prior art container-closure systems. As the cap is applied, the tamper-evident ring shifts laterally as illustrated in FIG. 17. The magnitude of this shift depends on clearance between the bottle finish and the cap and the magnitude of the bottle diameter. Contined application causes a part of the tamper-evident band to jump over the bead on the bottle producing the effect shown in FIG. 17b, that is cocking of the tamper-evident band and non-uniform deformation of the bridges. This deformation may result in bridge breakage during application and uneven fracturing of the bridges during removal even in cases where the bridges may be uniformly stressed during the removal process as described in more detail below. The action of the cap applied in this manner is known as "tire-on". Even though the mechanism of "tire-on" in some cases may reduce torque required to apply the "snap ring" closure, the ring shift produced by "tire-on", if excessive, can cause bridge break during application, the non-uniform straining of the bridge members and displacement of the ring into the cap in the manner illustrated in FIG. 17b. This latter effect can result in "application lock", which means that the torque sensitive chuck of the automatic assembly equipment releases the cap before complete application to the container.
Further, it has been observed that in prior art arrangements, upon removal of the cap, the interaction between the tamper-evident ring and the bottle finish bead is insufficient to produce the desired complete ring separation and retention on the container. Generally to achieve proper ring break away on initial removal, one designs a snap-ring closure of the type under consideration such that hoop forces maintained in expanding the tamper-evident ring over the bottle restriction are greater than the combined tensile strength of the bridges. Typically, the magnitude of these hoop forces can be determined on a torque gauge using unslit caps. Many factors can interfere and thwart this design criterion, such as the following: (1) poorly designed, minimum diameter bottle bead/maximum diameter cap rib which minimizes removal interference, possibly conbined with excessively strong bridges; (2) ductile material formulations which promote yielding and elasticity; (3) container finishes which do not allow the tamper-evident ring to relax sufficiently after application; and (4) container bead designs which do not permit proper cooperative interengagement of ring rib and bead during the bridge fracturing portion of the removal cycle.
In addition to the above, one other factor which has consistently thwarted the interference design criterion in prior art systems is the effect illustrated in FIG. 19 which is commonly encountered when the cap is threaded to a position where the bridges begin to break, and the cap and ring separate non-uniformly because the bridges do not break simultaneously. The tamper-evident ring assumes an undesirable angular relationship relative to a horizontal plane. When this happens, ultimate performance is highly erratic. The ring retention forces developed by interference of the container bead with the cap tamper-evident rib are dramatically decreased when the ring assumes the angular position shown in FIG. 19. In addition the ring is now permitted to pass over the container bead in a segmental, torsional motion known as "tiring off", approximately the reverse of the "tire on" mechanism discussed above. This motion is aptly named because it is the same mechanism by which a rubber tire is applied or removed from a rim. Tiring requires less force to move the ring over the container restriction than the uniform expansion desirable. The design interference criterion is thereby thwarted and the ring may come off the bottle finish still attached to the closure portion.